1. Field of the Invention
The invention relates to telecommunication systems. In particular, the invention relates to a method of sending packet-formed information optically, using different wavelengths of an optical cable, in such a way that one part of the wavelengths is used to transmit the address information of the packet and the part other is used to transmit the useful information. When packets are being routed, the router transmitting the packets in the telecommunication network reads only the address information on the address wavelength and sends the packets that came along with the useful wavelength further on based on the address.
2. Description of the Prior Art
The use of packet-switched telecommunication networks is becoming common because more and more formats of telecommunication, e.g. the traffic in accordance with the asynchronous transfer mode (ATM, Asynchronous Transfer Mode) and with the Internet protocol (IP, Internet Protocol), are being transferred in a packet-switched telecommunication network. Because of the packet-switched telecommunication networks, the same cable is used to transmit information of different formats, in which case less and less cables are needed and savings are made in the costs. Advancement in repeaters and routers enables building of faster and longer link connections.
Typically, telecommunication networks consist of link connections and routers. Usually it is the router that receives the packet, reads the address of the packet and sends the packet further on. If the packet has arrived at its destination, then the packet is taken off from the telecommunication network and is sent to the recipient.
One type of telecommunication networks form the optical cable networks. In the optical cable networks, true to the term, optical impulses are transmitted either in a glass or plastic conductor, which is called a fiber. The optical impulse is used to transport digitally coded information. It is possible to transmit the information in such a way that in a certain period of time the optical impulse means the same as bit 1, and the lacking of the optical impulse means the same as bit 0. Due to the longer spectral area of light, light may be used to transport more information than the traditional copper wire. Light is not sensitive to electromagnetic interference, either. Several telephone companies use optical fiber for the data transfer in long-distance lines. Since light weakens relatively fast in an optical cable, repeaters are needed in the optical cable line at regular intervals. Usually in an optical cable, information is being transferred in a digital form, in which case the advantages of digital data transfer are gained, such as a reliable and fast data transfer. Also the subtle attenuation in the optical cable and the large band width are the overpowering advantages of the optical cable as compared to copper wire. When speaking about an optical cable, the attenuation is used to mean the decrease of light output proceeding in the fiber of an optical cable.
The optical cable networks are divided into multi-mode and single-mode optical fiber networks. In the single-mode optical fiber networks, which are used in long-distance connections, only one beam of light is proceeding. In the multi-mode optical fiber networks, several beams of light are used with each of them having an individual angle of reflection. The beams of light in multi-mode optical fiber networks scatter faster than in a single-mode optical fiber network, and therefore the multi-mode optical fiber network is used only with short distances.
In wavelength division multiplexing, (WDM, Wavelength Division Multiplexing), in the same optical fiber, information is transmitted on different wavelengths. This makes it easy to increase the amount of the information to be transmitted in the fiber. For instance, if in a separate channel, information is being transported at the transfer rate of 2 Mbit/s, then the channels can be combined by wavelength division multiplexing, in which case it is possible to transfer 160 Mbit/s with a single optical fiber. Wavelength division multiplexing is sometimes called the dense wavelength division multiplexing (DWDM, Dense Wavelength Division Multiplexing).
When packet-formed information is being transmitted in a system multiplexed according to a wavelength in an optical fiber on just one wavelength, the header information, which is of a fixed form, is sent before the actual content of the packet. The header information is also separated from the actual content by a short guard time so that the router has time, based on the header information, to adjust the routing before the arrival of the useful information. The guard time is also used to prevent confusion, which might arise because different wavelengths use different rates. The packet is transmitted via the routing node based on the header information of the packet. If the packets arrive at the routing node via many different routes, then the packets have to be synchronized to certain periods. For routing, the router comprises a synchronizing device. In addition to the router, also buffering capacity is needed; several packets may not enter the same wavelength of the same link connection at the same time. To prevent the above-mentioned occurrence, optical buffering is needed. For optical data transmission, signal re-creation is needed in order to eliminate different signal mutilations, such as interfering signals, flutter and synchronization error (jitter).
Usually optical cable networks have been built between the exchanges of the telecommunication networks. Now with the Internet, the need for band width of digital services and telecommunication networks has increased, so the building of optical cable connections is getting closer to access networks.
Pulse code modulation (PCM, Pulse Code Modulation) is one mode of transferring analogously coded information digitally. In a transmitting analog signal, samples are taken according to a time slot defined by the sampling frequency. The amplitude of the samples is transformed into a binary number which may be represented by six bits. The bit encoded information is digitally transferred to the recipient, who reforms, based on the digital information, a new analog signal to be transmitted further on. Typically, an analog signal comprises voice, music or moving picture information. Usually in optical cables, information is transferred in a pulse code modulation form.
In a packet-switched data communication, the data to be transmitted, such as a file, is divided into small parts (packets) which are efficient to transfer and route in a telecommunication network. To the packets, an individualized serial number and address data are attached, after which the packets are transported by the telecommunication network. The packet-switched data transfer is connectionless, so no direct connection is established between the sender and the recipient, which is unlike a circuit-switched data transfer. In packet-switched data transfer, the packets are transferred along some telecommunication route to the recipient, in which case the packets may arrive at the destination also in an order which does not correspond to the original transmission order of packets. For arranging the packets in the right order, it is possible to use, e.g. the transfer control protocol (TCP, Transfer Control Protocol). When the packets are in the right order, the recipient combines the packets and receives the information to be transmitted. The Internet protocol or the asynchronous transfer mode are just one example of packet-switched traffic.
One problem is the rapid transmission of packet-formed information in an optical telecommunication network. In the optical telecommunication network, the packets are transformed in such a way that they can be transmitted by optical impulses. The sending and transferring of optical impulses succeeds relatively fast, but routing the optical impulses rapidly is difficult. If routing the telecommunication packets rapidly succeeds, then the output needed in the data transmission per telecommunication packet is lower, which saves cost.
There is no previous solution to the problems presented above. Previously, packet-formed information has been routed in such a way that on the same wavelength, both the address information and the useful information to be transported have been transmitted. Therefore, there has been a need for a guard time for the routing after the address information in order that the router has time to adjust the routing before the arrival of the information to be transported.